(1) Check the surface for defects such as decarburization and cracks before heat treatment. These surface defects will seriously reduce the fatigue limit of the spring.
(2) The quenching heating should pay special attention to prevent overheating and decarburization, do a good job in salt bath deoxidation, control the furnace gas atmosphere, and strictly control the heating temperature and time
(3) In order to reduce the deformation, the method of loading the spring during heating, the form of fixture and the method of quenching during cooling.
(4) Tempering as soon as possible after quenching, and heating as much as possible. Quick cooling after tempering can prevent temper brittleness and cause surface compressive stress, and improve fatigue strength.
Quality inspection of spring steel after heat treatment:
Before heat treatment–
(1) The rolled surface of steel is often the surface after the spring is made, so there should be no cracks, folds, scars, hair lines, bubbles, interlayers, and pressed oxide scales.
(2) Surface decarburization will significantly reduce the fatigue strength of the spring. The depth of the decarburized layer should be inspected as required.
After heat treatment–
(1) There should be no cracks, corrosive pitting and severe quenching deformation on the surface of the spring when observed with the naked eye or low-power magnifying glass.
(2) The hardness and its uniformity meet the requirements. In mass production, it is allowed to use a file to sample the hardness, but it must be noted that the position of the file mark should not affect the final accuracy of the spring.
(3) The metallographic structure should be troostite or a mixed organization of troostite and sorbite.
(4) After the leaf spring is assembled, the permanent deformation and static deflection test under working load are usually carried out. Comparison of fatigue properties of 50CrV steel valve spring after low temperature carbonitriding (soft nitriding) and ordinary heat treatment
Heat treatment process Breaking strain cycles Residual deformation after fracture (mm) Indoor atmospheric corrosion resistance test
Oil quenching at 860℃, tempering at 420℃ for 1h 1.78*10000 2.4 No oiling after blackening, rust spots appear after 14 days
Oil quenching at 860℃, soft nitriding at 480℃ for 3h >=100*10000 0.2 No rust for more than 850 days
Oil quenching at 860℃, soft nitriding at 510℃ for 3h >=100*10000 0.6 No rust for more than 850 days
Example; compression coil spring 50CrV 43-50HRC outer diameter ￠31.5 wire diameter ￠3.5 total height 50
Technological process; cold coil-shaping treatment-cutting correction-quenching-correction and tempering-inspection correction-final tempering-inspection, sandblasting, surface treatment.